1. Field of the Invention
The present invention relates to a process for manufacturing a sliding contact piece for medium to high current densities and more particularly to a process used especially to manufacture sliding contact pieces such as carbon brushes employed in electrical machines, especially in motor vehicles, where high current densities occur especially in starters.
2. Prior Art
So as to provide sliding contact pieces for medium to high current densities with favorable properties of both pure carbon contact pieces and metal contact pieces, such materials have long been combined in their manufacture; and in doing so what has been especially sought is an intimate connection of the carbon parts with the metal (DE 154 287 C).
Prior art also includes avoiding the use of additives of lead or antimony, which are contained in common sliding contact pieces and which provide a good cleaning action, cool the sliding contact piece in operation, and make it slide well against a mating contact, which, however, are toxic and harmful to the environment (EP 0525 222 A 1). To accomplish this, an admixture replacing the above additives has been separated from the copper by a layer to prevent alloying, which required special manufacturing measures. Lead substitutes or admixtures which are used are especially tin and/or zinc or an alloy thereof. The environmentally friendly admixtures, preferably tin or zinc, should not simply be mixed with the basic components (copper, graphite), since this would then produce an alloy which would be too hard for the desired purpose and would not have a low enough melting point.